1. Field of the Invention
The present invention is concerned with a process of producing a gypsum-based fiber-reinforced hardened body, and more particularly it relates to a process of producing such a hardened body in which gypsum (CaSO.sub.4.2H.sub.2 O) and slag are used as the main starting material and reinforcing fiber is incorporated therewith.
2. Description of the Prior Art
Hitherto, gypsum, slag, and a mixture thereof have been utilized as described below:
(1) Utilization of hemihydrate and II-type anhydrate: PA0 (2) Composite materials of hemihydrate and portland cement: PA0 (3) Sulfated slag cement:
The utilization of hemihydrate (CaSO.sub.4.1/2H.sub.2 O) and II-type anhydrate (CaSO.sub.4 is typified by a gypsum plaster board. That is, hemihydrate is obtained by dehydration of 3/2 mole of the bound water contained in gypsum (CaSO.sub.4.2H.sub.2 O). To hemihydrate are added fillers etc., and further 70% by weight or more of water is also added thereto, and the resulting mixture is kneaded and hardened to produce a board. As for II-type anhydrate, it is obtained by calcination of gypsum at about 600.degree. C. or more, and the obtained II-type anhydrate is ground. About 35% by weight of water is added to the anhydrate, and the mixture is kneaded and hardened to prepare a board.
These hardened gypsum bodies are obtained by utilizing the self-hardening property of hemihydrate and II-type anhydrate. The above mentioned methods of producing hardened gypsum bodies require the dehydration of gypsum, and in particular, a large quantity of heat is needed in obtaining II-type anhydrate.
Further, any of these hardened gypsum bodies are poor in water resistance. Besides, the hardened hemihydrate body is lower in strength since much water is required in obtaining such a hardened body. II-Type anhydrate is hardly susceptible of hydration even when water is added thereto. For the purpose of hydrating II-type anhydrate, it is necessary to add an accelerator. However, even when such an accelerator is added, the hydration cannot be completed in a short period and proceeds slowly. Because of this, the hardened II-type anhydrate body is liable to be expanded and destroyed in a long period of time, and therefore, it is poor in the stability required for general materials.
The composite method has been studied for the purpose of improving the water resistance of hardened gypsum bodies. However, the composite of hemihydrate and portland cement is disadvantageous in that the composite material exhibits too rapid a setting property and lacks workability. Owing to this, it is required to add a retarder. However, in some cases, the addition of a retarder results in reducing the strength of the final product to a great extent.
Moreover, after completion of hardening of the hemihydrate body, calcium aluminate (3CaO.Al.sub.2 O.sub.3) in the cement and the hemihydrate react with each other to form ettringite (3CaO.Al.sub.2 O.sub.3.3CaSO.sub.4. 31-32H.sub.2 O), and as a result, the hardened body is expanded and destroyed.
Sulfated slag cement is composed of 80 - 85% by weight of blast furnace slag, 10 - 15% by weight of anhydrate and about 5% by weight of portland cement. The latent hydraulic property of the blast furnace slag is utilized to produce a hardened body of sulfated slag cement. This hardened body is liable to be deteriorated due to an embrittlement phenomenon in its surface, which is most disadvantageous. Besides, when a large amount of II-type anhydrate is added to sulfated slag cement, the hardened body disadvantageously exhibits a similar property to that of the hardened bodies as mentioned above (2), and its deterioration is promoted.
In the foregoing conventional utilization of gypsum, cases (1) and (2) are based on the utilization of the self-hardening property possessed by hemihydrate and II-type anhydrate, and in case (3), the anhydrate is used as a stimulant for the hydration in a small amount.
As for gypsum, it does not have self-hardening properties, and therefore it has not become by itself the material to be utilized.
The inventors have studied to develop the use of gypsum. As the result, they have found that an unexpected hardened body having a rapid hardening property, high strength and water resistance is obtained by using gypsum (CaSO.sub.4.2H.sub.2 O) having no self-hardening property and slag having the latent hydraulic property as the main starting material and incorporating with the main starting material an alkali salt substance and an aluminum sulfate-containing substance. Also, it has been found that when reinforcing fiber is further used along with the main starting material and other ingredients as mentioned above, an improved hardened body can be obtained which is remarkably excellent in the physical properties including impact strength, change in length and particularly bending strength.